1. Field of the Invention
The present invention relates to a power supply device including a battery pack including a number of battery cells serially connected to each other, and in particular to a power supply device detecting voltages of the battery cells and safely charging/discharging the battery cells.
2. Description of the Related Art
A battery pack required to provide a large amount of output includes a number of battery cells serially connected to each other for increasing output voltage. Since the battery cells in this battery pack are serially connected to each other, the charging and discharging current values of all of the battery cells are same as each other. However, in the battery pack including a number of battery cells serially connected to each other, since the battery cells cannot have completely the same characteristics, the unbalance between their electrical characteristics may bring a particular battery cell to be over-charged or over-discharged. If a battery cell is over-charged or over-discharged, the battery cell will remarkably deteriorate, and may be brought into a dangerous condition. In order to prevent this, in a battery pack including a number of battery cells serially connected to each other, the voltages of the battery cells are detected so that the battery cells are charged/discharged under control. For example, in the case where the battery pack is charged, if the voltage of a particular battery cell becomes higher than a predetermined maximum voltage, the battery pack stops being charged. On the other hand, in the case where the battery pack is discharged, if the voltage of a particular battery cell becomes lower than a predetermined minimum voltage, the battery pack stops being discharged. Thus, the battery pack is charged/discharged with the battery cells being protected.
In a power supply device charging/discharging the battery pack with the battery cells being protected, the power supply device includes a voltage detecting circuit that detects the voltages of the battery cells, and controls charging/discharging operation based on the voltages of the battery cells detected by this voltage detecting circuit. The voltage detecting circuit is connected to the positive and negative sides of the battery cells through voltage detection lines. In the case where a power supply device includes a number of battery cells serially connected to each other, since such a voltage detecting circuit is connected to the battery cells through a number of voltage detection lines, if any of voltage detection lines is disconnected, the voltage detecting circuit cannot properly detect the voltages of the battery cells.
In a power supply device including a number of battery cells serially connected to each other, the power supply device is composed of a plurality of battery units 90, as shown in FIG. 1 (see Japanese Patent Laid-Open Publication No. JP 2010-81692 A). Each of the battery units 90 includes a battery block 92, a voltage detecting circuit 94, and a power supply circuit 95. The battery block 92 includes a plurality of battery cells 93 serially connected to each other. The voltage detecting circuit 94 detects the voltages of the battery cells 93, which compose corresponding one of the battery blocks 92. The power supply circuit 95 supplies electric power to a power supply of the voltage detecting circuit 94. The plurality of battery blocks 92 are connected serially to each other, and compose a battery pack 91.
In this illustrated power supply device, each of the battery units 90 includes the voltage detecting circuit 94 for detecting the voltages of the battery cells 93, which compose corresponding one of the battery blocks 92. When operated, each of the voltage detecting circuits 94 is supplied with electric power from corresponding one of the battery blocks 92. For example, DC/DC converters are used as the power supply circuits 95, which supply electric power for operating the voltage detecting circuits 94 from the battery blocks 92. In this illustrated power supply device, which includes the battery pack 91 composed of the plurality of battery blocks 92 serially connected to each other, the remaining capacities of the battery blocks 92 will be unbalanced. The unbalance between the remaining capacities increases the probability that a particular battery block 92 will be over-charged or over-discharged, which in turn may cause battery deterioration. For this reason, in order to eliminate the unbalance, equalizing circuits 97 are connected to the battery blocks 92. That is, the known power supply device is designed to reduce the unbalance between the battery blocks 92.
However, in the power supply device, which includes the battery pack 91 composed of the plurality of battery blocks 92 serially connected to each other, the unbalance between the currents of the power supply circuits 95 of the battery units 90 makes it difficult to detect disconnection of the voltage detection lines 96. In the normal condition, if disconnection occurs in the voltage detection line 96 through which current flows, the current stops flowing through the voltage detection line 96, which in turn changes the detection voltage detected by the voltage detecting circuit 94. Thus, in the case where a current flows through the voltage detection line 96 in the normal condition, it is possible to detect disconnection of the voltage detection line 96. However, in the case where a current does not flow through the voltage detection line 96 in the normal condition, the current value in the voltage detection line 96 does not change even if disconnection occurs in the voltage detection line 96. Accordingly, it is impossible to detect disconnection of the voltage detection line 96. The reason why disconnection of the voltage detection line 96 cannot be detected in the case where a current does not flow through the voltage detection line 96 in the normal condition is that the condition where a current does not flow in a line is the same as condition where a resistor with infinite resistance is connected on the line.
In the power supply device, which includes plurality of battery blocks 92 serially connected to each other as shown in FIG. 1, a current will not flow through one of the voltage detection lines 96 if the power supply circuits 95 of the battery blocks 92 have particular amounts of consumption currents. For this reason, disconnection of the one of the voltage detection lines 96 cannot be detected through which the current does not flow.
FIGS. 2 and 3 show this reason. In a power supply device shown in FIG. 2 includes a battery pack 71 composed of three battery blocks 72, which are serially connected to each other. Each of the battery blocks 72 is composed of six battery cells 73, which are serially connected to each other. In the circuit construction shown in FIG. 2, input resistors 79 are connected to the battery cells 73. Accordingly, an input current (Id) flows in the battery cell 73 through the input resistor 79. In addition, consumption currents (Ip) of power supply circuits 75 flow in the battery blocks 72. The power supply circuits 75 supply electric power to voltage detecting circuits 74. DC/DC converters are used as the power supply circuits 75. The consumption currents of the power supply circuits 75 correspond to consumption currents (Ip) of the DC/DC converters. The consumption current values of DC/DC converters cannot be completely the same as each other. For this reason, consumption currents are unbalanced.
FIG. 3(1) indicates the values of currents, which flow in the battery cells 73, in the condition where consumption currents (Ip) of the power supply circuits 75 of the first and second battery units 70A and 70B are the same current value of 10 mA. FIG. 3(2) indicates current values, which flow in the battery cells 73, in the unbalanced condition where the consumption currents (Ip) of the power supply circuits 75 of the first and second battery units 70A and 70B are 10 mA and 10.05 mA, respectively. A current flows in the voltage detection line 76. The amount of this current corresponds to the current difference between currents flowing in the adjacent battery cells 73. The reason is that the current difference between currents flowing in the adjacent battery cells 73 is provided by the current flowing in the voltage detection line 76. Accordingly, if currents of the same current value flow in the adjacent battery cells 73, a current does not flow in the voltage detection line 76.
The amount of a current gets larger which flows in one of the battery cells 73 as the one of the battery cells 73 is closer to an earth line 76A. The reason is that is input currents (Id) flow as shown by the arrows in FIG. 2. Since consumption currents (Ip) in FIG. 3(1) of the power supply circuits 75 are the same, all of current values of adjacent battery cells 73 are different from each other. Accordingly, disconnection of all of the voltage detection lines 76 can be detected.
However, as shown in FIG. 3 (2), in the case where the power consumption values of the power supply circuits 75 are different from each other, the current values of the battery cells 73a and 73b on the connection side between the battery blocks 72 of the first and second battery units 70A and 70B may be the same value of 10.06 mA. In this case, a current does not flow in the voltage detection line 76 connected to nodes 86 between these battery cells 73a and 73b. For this reason, disconnection of the voltage detection line cannot be detected.
The present invention has been developed for solving the aforementioned disadvantages. It is an important object of the present invention is to provide a power supply device capable of surely detecting disconnection of all of voltage detection lines and the voltages of battery cells.